No, I'm pretty sure that there is no age limit but you wouldn't really see some kid in elementary school take it.

When I took it last december, I saw people ranging in age from what seemed like 14-60. There were hispanics, african americans, caucasians, asians, and I even saw a few arabs. There were middle aged business men and I swear I think I saw some otaku there as well.

So in short, there is no age limit, practically every one from any (reasonable) age group takes the test and just for the sake of this post all different ethnic groups participate as well.

I was actually rather suprised to see just how many different groups of people are interested in learning Japanese.

Two muffins were baking in an oven. One turns to the other and says "sure is hot in here." The other replies "AH TALKING MUFFIN!"